This invention relates to monolithic optical circuits of the type disclosed in a copending application Ser. No. 138,165 filed Apr. 7, 1980 by Vincent L. Evanchuk, (now abandoned), and more particularly to an improved way of providing crossing light conductors on the same grade or level.
It is usually impossible to design a circuit on a single plane on which all conductors can be kept separate, without intersecting or crossing over, especially when the circuit elements to be connected have a plurality of terminals. In electrical conductive printed circuits the necessary crossovers are fabricated by adding additional layers of circuitry and connections are made between layers by plating through drilled holes at appropriate points. In a monolithic optical circuit structure proposed by Vincent L. Evanchuk in the aforesaid copending application filed concurrently herewith, crossovers are fabricated by depositing additional layers of radiation sensitive plastic, exposing the plastic through a mask, and washing away the unexposed, uncatalyzed plastic-leaving elevated crossovers connected to conductors in lower layers. If even only one crossover on a monolithic optical circuit board is needed, two layers are required; then the number of process steps is roughly doubled. This has a direct effect on cost. In certain cases it would be desirable to eliminate the need for additional levels in providing a crossing between light conductors.
In the aforesaid Evanchuk application, a keyboard is fabricated by forming a monolithic plastic block with internal light conductors brought up through it to "keyholes" at the keyboard surface from light distribution conductors formed in a first level on a substrate. The illuminating light is brough up to each "keyhole" by a rising conductor illuminated through a source conductor to join a second rising conductor that connects with a sensing conductor; these rising conductors join at the keyboard surface in an inverted V to form the keyhole. The departure of the rising conductors from normal is less than the critical angle of refraction so that the illuminating light in one conductor passes out to the ambient atmosphere unless a finger or other reflecting object is placed directly on the keyhole to reflect the light beam back down into the sensing conductor.
To insure proper registration between the distribution circuit board and the rising conductors, and to insure a proper joining of the rising conductors, high precision is required. The angle of the irradiating beam used in fabricating the rising conductors must be very carefully controlled throughout the entire sequence of steps used in fabricating the plurality of rising conductors, as must its location at each exposure, since this is, in practice, a step-by-step operation of one exposure per rising conductor. Manufacture would be slow and costly. This would be especially true in any application which requires a large number of illumination points, such as a large keyboard with a mosaic of hundreds of keyholes to be used for various purposes.
An object of this invention is to fabricate monolithic optical circuit boards in layers that are perpendicular to its upper surface, each layer containing a plurality of keyholes illuminated from source light entering at one end through a conductor that branches.